It's only really necessary to discard during the development stage of a starter. This has two effects, it stops the starter getting huge before it gets to the stage of being usefully active and balanced and it helps to establish the right balance of yeasts and bacteria (or alternatively helps to discourage the ones that you don't want).

Once you have a stable and balanced starter there shouldn't be any need to discard. You take what you need to start the levain for your loaf and you replace what you take with flour and water to keep your stock happy and well fed. You have to add water to keep the hydration of the starter at or about your desired consistency whether it be fluid or stiff.

I use a desem style starter, basically a very thick normal starter that you just refresh after use with two thirds water and one third flour, then keep in the fridge. This way has no waste and is pretty stable & easy to maintain.
NB check out sourdoughbaker.com.au for some more info!

the starter will grow, and grow, and grow into a huge mass in a short time without discarding some. but if baking often enough, you are always removing some. that can be balanced with your baking schedule. I'll bake 2-3 times a week. starting with about 100 grams starter, I'll feed about 100 grams over a couple days to get enough starter for the bake. after removing what's needed, the feeding schedule starts again, and I have enough for the next bake. lather, rinse, repeat! just adding flour makes a very thick starter, which is preferred by some. thicker will generally slow things down a tad, makes it harder for the bugs to get to food. thinner can have the opposite effect. as temps get warmer, I go on the thicker side to slow things down so I don't have to feed as often - trying to keep it at once a day instead of twice daily. last time I discarded starter was when it first got going, that was something like 4 months ago, but I like fresh bread and bake often enough. happy baking!